As effects of catechins, there have been reported a suppressing effect on the increase in cholesterol level and an inhibitory effect on α-amylase activities. For such physiological effects to emerge, an adult is required to drink 4 to 5 cups of tea in a day. Accordingly, there has been a demand for the technology enabling catechins to be contained in a beverage at a high concentration so that a large amount of catechins can be ingested conveniently. As one of the methods for this, catechins of a dissolved form prepared from a concentrate, purified product or the like of a green tea extract can be added to a beverage.
However, a beverage with a concentrate or purified product of a green tea extract added therein tends to develop grouts and precipitates when stored for a long time after its production, and consequently largely damages its external appearance. Conventional tea beverages, especially green tea beverages, may also develop grouts and precipitates during storage, but in the case of beverages with a concentrate or purified product of a green tea extract added therein, their external appearance severely deteriorates. The grouts and precipitates in green tea beverages are considered to be caused by the formation of complexes from components such as polysaccharides, proteins, polyphenols, metal ions and the like. The mechanisms of formation of grouts and precipitates are complex, and a variety of countermeasures have been considered to date. As countermeasures focused on high-molecular components in green tea, there have been methods for suppressing the formation of grouts by causing components of a high molecular complex to be disintegrated into lower molecular weight substances by enzymatic treatment (JP-A-05-328901, JP-A-11-308965); and a method for suppressing the formation of grouts by fractionating green tea components through an ultrafiltration membrane to substantially eliminate high molecular substances whose molecular weights are 10,000 and higher (JP-A-04-045744). As a countermeasure focused on metal ions, on the other hand, there has been a method for reducing a light blue color and turbidity by a treatment with a cation exchange resin, followed by nanofiltration (JP-A-11-504224).